Chemical analysis using liquid reagents is widely practiced in many fields, and most especially in medical diagnosis. The term "liquid reagents" encompasses reagents which in their pure or normal state might not be liquid but which have been rendered liquid in some manner such as by dissolving the reagent in a suitable carrier.
Typically, in chemical analysis a sample of the substance to be analysed is placed, sometimes following dilution, in a receptacle commonly known as a cuvette. Measured quantities of one or more liquid reagents are added to the sample in the cuvette and the resulting mixture is then monitored for a change in some selected property. In theory, almost any property could be monitored, but for reasons of practical convenience the property monitored is almost always an optical property. Commonly the mixture will be monitored for a change in colour (i.e. a change in the degree to which one or more specific wavelengths of light are absorbed or transmitted). Other optical properties which may be monitored are turbidity and fluorescence. Non-optical properties which may be monitored are electrical conductivity and pH. In all cases, the chemical reagent or reagents chosen and the property monitored are selected so that the presence or absence of a chemical or type of chemical in the sample can be detected by the presence or absence or the nature of a change in the monitored property of the mixture.
Presently available analytical equipment normally uses peristaltic or piston pumps to deliver the reagents to the cuvette containing the sample. However, these pumps are unreliable and may suffer from leaking seals and split pump tubes. Additionally, the analysers suffer from poor reproducibility of analytical results because the pumps cannot be highly accurate in the quantities of reagents delivered.
A further requirement for accurate analysis is that the liquid reagent should be of good quality and uncontaminated. It is increasingly common for liquid reagents to be pre-packed in bottles which allow improved quality control and security and minimises contamination during storage. When needed, a bottle of a particular reagent is opened and provided as a reservoir for a pump in the analyser apparatus. However, once opened the bottles are subject to contamination if the entire contents of the bottle is not used immediately.
The pumps increase the weight and bulk of the apparatus, and normally need to be driven by motors powered by main electricity. This limits the degree to which such apparatus is portable. Additionally, the pump motors are a source of heat whereas many chemical reagents are heat sensitive. Accordingly, if heat from the pump motors is not to cause deterioration of the reagents the apparatus must be designed to hold the reagent reservoirs or bottles spaced from the pump motors, which tends to increase the size of the instruments and also increases the length of pump tubing which must be employed.